Methods for electrosurgical tissue treatment in conductive fluid

ABSTRACT

An electrosurgical probe ( 10 ) comprises a shaft ( 13 ) having an electrode array ( 58 ) at its distal end and a connector ( 19 ) at its proximal end for coupling the electrode array to a high frequency power supply ( 28 ). The shaft includes a return electrode ( 56 ) recessed from its distal end and enclosed within an insulating jacket ( 18 ). The return electrode defines an inner passage ( 83 ) electrically connected to both the return electrode and the electrode array for passage of an electrically conducting liquid ( 50 ). By applying high frequency voltage to the electrode array and the return electrode, the electrically conducting liquid generates a current flow path between the return electrode and the electrode array so that target tissue may be cut or ablated. The probe is particularly useful in dry environments, such as the mouth or abdominal cavity, because the electrically conducting liquid provides the necessary return current path between the active and return electrodes.

This application is a continuation of and claims the benefit of U.S.application Ser. No. 09/262,281 filed Mar. 4, 1999 which is a divisionof Ser. No. 09/098,205, filed Jul. 27, 1998, now U.S. Pat. No. 6,224,590which is a division of Ser. No. 08/795,686, filed Feb. 5, 1997, now U.S.Pat. No. 5,871,469, which is a division of Ser. No. 08/561,958 filedNov. 22, 1995, now U.S. Pat. No. 5,697,882, which is a CIP of Ser. No.08/485,219 filed Jun. 7, 1995, now U.S. Pat. No. 5,697,281, which is aCIP of Ser. No. 08/446,767 filed Jun. 2, 1995, now U.S. Pat. No.5,687,909, which is a CIP of Ser. No. 08/059,681 filed May 10, 1993, nowabandoned, the disclosure of which is incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates generally to the field of electrosurgeryand, more particularly, to surgical devices and methods which employhigh frequency voltage to cut and ablate tissue.

The field of electrosurgery includes a number of loosely relatedsurgical techniques which have in common the application of electricalenergy to modify the structure or integrity of patient tissue.Electrosurgical procedures usually operate through the application ofvery high frequency currents to cut or ablate tissue structures, wherethe operation can be monopolar or bipolar. Monopolar techniques rely onexternal grounding of the patient, where the surgical device definesonly a single electrode pole. Bipolar devices comprise both electrodesfor the application of current between their surfaces.

Electrosurgical procedures and techniques are particularly advantageoussince they generally reduce patient bleeding and trauma associated withcutting operations. Current electrosurgical device and procedures,however, suffer from a number of disadvantages. For example, monopolardevices generally direct electric current along a defined path from theexposed or active electrode through the patient's body to the returnelectrode, which is externally attached to a suitable location on thepatient. This creates the potential danger that the electric currentwill flow through undefined paths in the patient's body, therebyincreasing the risk of unwanted electrical stimulation to portions ofthe patient's body. In addition, since the defined path through thepatient's body has a relatively high impedance (because of the largedistance or resistivity of the patient's body), large voltagedifferences must typically be applied between the return and activeelectrodes in order to generate a current suitable for ablation orcutting of the target tissue. This current, however, may inadvertentlyflow along body paths having less impedance than the defined electricalpath, which will substantially increase the current flowing throughthese paths, possibly causing damage to or destroying tissue along andsurrounding this pathway.

Bipolar electrosurgical devices have an inherent advantage overmonopolar devices because the return current path does not flow throughthe patient. In bipolar electrosurgical devices, both the active andreturn electrode are typically exposed so that they may both contacttissue, thereby providing a return current path from the active to thereturn electrode through the tissue. One drawback with thisconfiguration, however, is that the return electrode may cause tissuedesiccation or destruction at its contact point with the patient'stissue. In addition, the active and return electrodes are typicallypositioned close together to ensure that the return current flowsdirectly from the active to the return electrode. The close proximity ofthese electrodes generates the danger that the current will short acrossthe electrodes, possibly impairing the electrical control system and/ordamaging or destroying surrounding tissue.

The use of electrosurgical procedures (both monopolar and bipolar) inelectrically conductive environments can be further problematic. Forexample, many arthroscopic procedures require flushing of the region tobe treated with isotonic saline (also referred to as normal saline),both to maintain an isotonic environment and to keep the field ofviewing clear. The presence of saline, which is a highly conductiveelectrolyte, can also cause shorting of the electrosurgical electrode inboth monopolar and bipolar modes. Such shorting causes unnecessaryheating in the treatment environment and can further cause non-specifictissue destruction.

Many surgical procedures, such as oral, laparoscopic and open surgicalprocedures, are not performed with the target tissue submerged under anirrigant. In laparoscopic procedures, such as the resection of the gallbladder from the liver, for example, the abdominal cavity is pressurizedwith carbon dioxide (pneumoperitoneum) to provide working space for theinstruments and to improve the surgeon's visibility of the surgicalsite. Other procedures, such as the ablation of muscle or gingiva tissuein the mouth, the ablation and necrosis of diseased tissue, or theablation of epidermal tissue, are also typically performed in a “dry”environment or field (i.e., not submerged under an electricallyconducting irrigant).

Present electrosurgical techniques used for tissue ablation also sufferfrom an inability to control the depth of necrosis in the tissue beingtreated. Most electrosurgical devices rely on creation of an electricarc between the treating electrode and the tissue being cut or ablatedto cause the desired localized heating. Such arcs, however, often createvery high temperatures causing a depth of necrosis greater than 500 μm,frequently greater than 800 μm, and sometimes as great as 1700 μm. Theinability to control such depth of necrosis is a significantdisadvantage in using electrosurgical techniques for tissue ablation,particularly in arthroscopic procedures for ablating and/or reshapingfibrocartilage, articular cartilage, meniscal tissue, and the like.

In an effect to overcome at least some of these limitations ofelectrosurgery, laser apparatus have been developed for use inarthroscopic and other procedures. Lasers do not suffer from electricalshorting in conductive environments, and certain types of lasers allowfor very controlled cutting with limited depth of necrosis. Despitethese advantages, laser devices suffer from their own set ofdeficiencies. In the first place, laser equipment can be very expensivebecause of the costs associated with the laser light sources. Moreover,those lasers which permit acceptable depths of necrosis (such as eximerlasers, erbium:YAG lasers, and the like) provide a very low volumetricablation rate, which is a particular disadvantage in cutting andablation of fibrocartilage, articular cartilage, and meniscal tissue.The holmium:YAG and Nd:YAG lasers provide much higher volumetricablation rates, but are much less able to control depth of necrosis thanare the slower laser devices. The CO₂ lasers provide high rate ofablation and low depth of tissue necrosis, but cannot operate in aliquid-filled cavity.

For these and other reasons, improved systems and methods are desiredfor the electrosurgical ablation and cutting of tissue. These systemsand methods should be capable of selectively cutting and ablating tissueand other body structures in electrically conductive environments, suchas regions filled with blood or irrigated with electrically conductivesolutions, such as isotonic saline, and in relatively dry environments,such as those encountered in oral, dermatological, laparoscopic,thoracosopic and open surgical procedures. Such apparatus and methodsshould be able to perform cutting and ablation of tissues, whilelimiting the depth of necrosis and limiting the damage to tissueadjacent to the treatment site.

2. Description of the Background Art

Devices incorporating radio frequency electrodes for use inelectrosurgical and electrocautery techniques are described in Rand et.al. (1985) J. Arthro. Surg. 1:242-246 and U.S. Pat. Nos. 5,281,216;4,943,290; 4,936,301; 4,593,691; 4,223,800; and 4,202,337. U.S. Pat.Nos. 4,943,290 and 4,036,301 describe methods for injectingnon-conducting liquid over the tip of a monopolar electrosurgicalelectrode to electrically isolate the electrode, while energized, from asurrounding electrically conducting irrigant. U.S. Pat. Nos. 5,195,959and 4,674,499 describe monopolar and bipolar electrosurgical devices,respectively, that include a conduit for irrigating the surgical site.

U.S. Pat. Nos. 5,217,455, 5,423,803, 5,102,410, 5,282,797, 5,290,273,5,304,170, 5,312,395, 5,336,217 describe laser treatment methods forremoving abnormal skin cells, such as pigmentations, lesions, softtissue and the like. U.S. Pat. Nos. 5,445,634 and 5,370,642 describemethods for using laser energy to divide, incise or resect tissue duringcosmetic surgery. U.S. Pat. No. 5,261,410 is directed to a method andapparatus for detecting and removing malignant tumor tissue. U.S. Pat.Nos. 5,380,316, 4,658,817, 5,389,096, PCT application No. WO 94/14383and European Patent Application No. 0 515 867 describe methods andapparatus for percutaneous myocardial revascularization. These methodsand apparatus involve directing laser energy against the heart tissue toform transverse channels through the myocardium to increase blood flowfrom the ventricular cavity to the myocardium.

SUMMARY OP THE INVENTION

The present invention provides a system and method for selectivelyapplying electrical energy to structures within or on the surface of apatient's body. The system and method allow the surgical team to performelectrosurgical interventions, such as ablation and cutting of body,structures, while limiting the depth of necrosis and limiting damage totissue adjacent the treatment site. The system and method of the presentinvention are useful for surgical procedures in relatively dryenvironments, such as treating and shaping gingiva, for tissuedissation, e.g. separation of gall bladder from th liver, ablation andnecrosis or diseased tissue, such as fibroid tumors, and dermatologicalprocedures involving surface tissue ablation on the epidermis, such asscar or tattoo removal, tissue rejuvenation and the like. The presentinvention may also be useful in electrically conducting environments,such as arthroscopic or cystoscopic surgical procedures. In addition,the present invention is useful for canalizing or boring channels orholes through tissue, such as the ventricular wall of the heart duringtransmyocardial revascularization procedures.

The method of the present invention comprises positioning anelectrosurgical probe adjacent the target tissue so that at least oneactive electrode is brought into close proximity to the target site. Areturn electrode is positioned within an electrically conducting liquid,such as isotonic saline, to generate a current flow path between thetarget site and the return electrode. High frequency voltage is thenapplied between the active and return electrode through the current flowpath created by the electrically conducting liquid in either a bipolaror monopolar manner. The probe may then be translated, reciprocated orotherwise manipulated to cut the tissue or effect the desired depth ofablation.

The current flow path may be generated by submerging the tissue site inan electrical conducting fluid (e.g., arthroscopic surgery and the like)or by directing an electrically conducting liquid along a fluid pathpast the return electrode and to the target site to generate the currentflow path between the target site and the return electrode. This lattermethod is particularly effective in a dry environment (i.e., the tissueis not submerged in fluid), such as open, endoscopic or oral surgery,because the electrically conducting liquid provides a suitable currentflow path from the target site to the return electrode. The activeelectrode is preferably disposed at the distal end of the probe and thereturn electrode is spaced from the active electrode and enclosed withinan insulating sheath. This minimizes exposure of the return electrode tosurrounding tissue and minimizes shorting of the current between theactive and return electrodes. In oral procedures, the probe may beintroduced directly into the cavity of the open mouth so that the activeelectrode is positioned against gingival or mucosal tissue. Inendoscopic procedures, the probe will typically be passed through aconventional trocar cannula while viewing of the operative site isprovided through she use of a laparoscope disposed in a separatecannula.

In a specific aspect of the invention, the high frequency voltageapplied between the active and return electrodes generates high voltagegradients in the vicinity of the probe tip. These high voltage gradientsare sufficient to create an electric field at the distal boundary of theactive electrode(s) that is sufficiently high to break down the tissuethrough molecular dissociation or disintegration. The high frequencyvoltage imparts energy to the target site to ablate a thin layer oftissue without causing substantial tissue necrosis beyond the boundaryof the thin layer of tissue ablated. This ablative process can beprecisely controlled to effect the volumetric removal of tissue as thinas a few layers of cells with minimal heating of or damage tosurrounding or underlying tissue structures.

Applicants believe that this precisely controlled ablation is at leastpartly caused by the high electric field generated around the tip of theactive electrode(s) within the electrically conductive liquid. Theelectric field vaporizes the electrically conductive liquid into a thinlayer over at least a portion of the active electrode surface and thenionizes the vapor layer due to the presence of an ionizable specieswithin the liquid. This ionization and the presence of high electricfields in a low density vaporized layer induces the discharge of highlyenergetic electrons and photons in the form of ultraviolet energy fromthe vapor layer. The ultraviolet energy and/or energetic electrons causedisintegration of the tissue molecules adjacent to the vapor layer. Thisenergy discharge can be precisely controlled to effect the volumetricremoval of tissue thicknesses ranging from millimeters to a few layersof cells without heating or otherwise damaging surrounding or underlyingcell structures.

The active electrode(s) will be spaced away from the target tissue by asuitable distance during the ablation process. This spacing allows forthe continual resupply of electrically conducting liquid at theinterface between the active electrode(s) and the target tissue surface.This continual resupply of the electrically conducting liquid helps toensure that the thin vapor layer or region will remain over at least aportion of the active electrode(s) between the active electrode(s) andthe tissue surface. Preferably, the active electrode(s) will betranslated and/or rotated transversely relative to the tissue, i.e., ina light brushing motion, to maintain the supply of electricallyconducting fluid in the region between the active electrode(s) and thetissue. This dynamic movement of the active electrode(s) over the tissuesite also allows the electrically conducting liquid to cool the tissuesurrounding recently ablated areas to minimize damage to thissurrounding tissue.

The apparatus according to the present invention comprises anelectrosurgical probe having a shaft with a proximal end, a distal end,and at least one active electrode at or near the distal end. A connectoris provided at or near the proximal end of the shaft for electricallycoupling the active electrode to a high frequency voltage source. Areturn electrode coupled to the voltage source is spaced a sufficientdistance from the active electrode to substantially avoid or minimizecurrent shorting therebetween and, in dry environments, to shield thereturn electrode from tissue at the target site of ablation or from thesurgeon. In irrigant flooded environments, such as arthroscopic surgery,the area of the return electrode is sufficiently large to result in lowcurrent densities that effectively preclude damage to nearby tissue. Thereturn electrode may be provided integral with the shaft of the probe orit may be separate from the shaft (e.g., on a liquid supply instrument).In both cases, the return electrode defines an inner, annular surface ofthe pathway for flow of electrically conducting liquid therethrough. Theliquid is directed past the surface of the return electrode and over theactive electrode to thereby provide a return current flow path betweenthe target tissue site and the return electrode.

The active and return electrodes will preferably be configured suchthat, upon the application of a sufficient high-frequency voltage, athin layer of the electrically conducting layer is vaporized over atleast a portion of the active electrode(s) in the region between theactive electrode(s) and the target tissue. To accomplish this, theactive electrode(s) will be configured such that high electric fielddensities form at the distal tips of the active electrode(s). By way ofexample, the present invention may utilize an electrode array ofelectrode terminals flush with or recessed from or extending from thedistal end of the probe. The electrode terminals will preferably have asufficiently small area, extension (or recession) length from the probeand sharp edges and/or surface asperities such that localized highcurrent densities are promoted on the electrode terminals which, inturn, lead to the formation of a vaporized layer or region over at leasta portion of the active electrode(s) followed by the high electric fieldinduced breakdown (i.e., ionization) of ionizable species within thevapor layer or region and the emission of photon and/or electrons ofsufficient energy to cause dissociation of molecules within the targettissue.

In an exemplary embodiment, the active electrode(s) are sized andarranged to create localized sources of energy (e.g., point sources orsources with a relatively small effective radius) at the distal tips ofthe electrode(s) when a sufficiently high frequency voltage is appliedto the return and active electrodes. These small localized sourcesgenerate intense energy at the distal ends of the electrodes formolecular dissociation or ablation of tissue in contact with or in closeproximity to the electrode tips. In addition, since the localizedsources have relatively small radii, the energy flux decreases with thesquare of the distance from the localized sources so that the tissue atgreater distances from the electrode tips are not significantly affectedby the energy flux.

A further understanding of the nature and advantages of the inventionwill become apparent by reference to the remaining portions of thespecification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the electrosurgical system including anelectrosurgical probe, an electrically conducting liquid supply and anelectrosurgical power supply constructed in accordance with theprinciples of the present invention;

FIG. 2A is an enlarged, cross-sectional view of the distal tip of theelectrosurgical probe of FIG. 1 illustrating an electrode arrangementsuitable for rapid cutting and ablation of tissue structures;

FIG. 2B is an enlarged end view of the distal tip of the electrosurgicalprobe of FIG. 1;

FIG. 2C is a cross-sectional view of the proximal end of theelectrosurgical probe, illustrating an arrangement for coupling theprobe to the electrically conducting liquid supply of FIG. 1;

FIG. 3 is a detailed cross-sectional view of an alternative embodimentof the electrosurgical probe of FIG. 1;

FIG. 4 is an end view of the distal end of the electrosurgical probe ofFIG. 3;

FIG. 5 is an end view of an another embodiment of the electrosurgicalprobe of FIG. 1;

FIG. 6 is a partial cross-sectional side view of a further embodiment ofthe electrosurgical probe with the electrode array disposed transverselyto the axis of the probe;

FIG. 7 is a partial front cross-sectional view of an electrosurgicalprobe and an electrically conductive liquid supply shaft illustratinguse of the probe and the shaft in ablating target tissue;

FIG. 8 is an enlarged, cross-sectional view of the distal tip of yetanother embodiment of the electrosurgical probe of FIG. 1;

FIG. 9 is a detailed end view or the probe of FIG. 8;

FIG. 10 is a side view of an electrosurgical probe having a shaft withan angled distal coation;

FIG. 11 is a side view of an electrosurgical probe having a shaft with aperpendicular distal portion;

FIG. 12 is a schematic view of an electrosurgical probe having twoscrewdriver-shaped electrodes extending from the distal end;

FIG. 13 is an end view of the probe of FIG. 12;

FIG. 14 illustrates use of the probe of FIG. 12 for the rapid cutting oftissue;

FIG. 15 is a cross-sectional view of the distal tip of theelectrosurgical probe, illustrating electric field lines between theactive and return electrodes;

FIG. 16 is an enlarged cross-sectional view of the distal tip of theprobe of FIG. 15, illustrating a vapor layer formed between the activeelectrodes and the target tissue;

FIG. 17 is a cross-sectional view of an alternative electrosurgicalprobe for applying high frequency voltage to epidermal tissue layers;

FIG. 18 is a sectional view of the human heart, illustrating theelectrosurgical probe within the ventricular cavity for performing atransmyocardial revascularization procedure;

FIG. 19 is a cross-sectional view of the probe boring a channel throughthe ventricular wall;

FIG. 20 depicts an alternative embodiment of the probe of FIG. 19 havingan inner lumen for aspirating fluid and gases from the transmyocardialchannel;

FIG. 21 depicts a distal portion of an alternative embodiment of theprobe of FIGS. 2A-2C incorporating a single electrode with a tubulargeometry;

FIG. 22 is a cross-sectional view of the distal end of the probe of FIG.21;

FIG. 23 is a side cross-sectional view of a distal portion of a furtherembodiment of the probe of FIGS. 2A-2C incorporating a multiplicity ofelectrodes which converge to a single electrode lead; and

FIG. 24 is a side cross-sectional view of a distal portion of yetanother embodiment of the probe of FIGS. 2A-2C incorporating a singleelectrode connected to a single electrode lead.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention provides a system and method for selectivelyapplying electrical energy to a target location within or on a patient'sbody, such as solid tissue or the like, particularly including gingivaltissues and mucosal tissues located in the mouth or epidermal tissue onthe outer skin. In addition, tissues which may be treated by the systemand method of the present invention include tumors, abnormal tissues,and the like. The invention may also be used for canalizing or boringchannels or holes through tissue, such as the ventricular wall duringtransmyocardial revascularization procedures. For convenience, theremaining disclosure will be directed specifically to the cutting,shaping or ablation of gingival or mucosal tissue in oral surgicalprocedures, the surface tissue ablation of the epidermis indermatological procedures and the canalization of channels through themyocardium of the heart, but it will be appreciated that the system andmethod can be applied equally well to procedures involving other tissuesof the body, as well as to other procedures including open surgery,laparoscopic surgery, thoracoscopic surgery, and other endoscopicsurgical procedures.

In addition, the present invention is particularly useful in procedureswhere the tissue site is flooded or submerged with an electricallyconducting fluid, such as isotonic saline. Such procedures, e.g.,arthroscopic surgery and the like, are described in detail in co-pendingPCT International Application, U.S. National Phase Serial No.PCT/US94/C5168, filed on May 10, 1994, the complete disclosure of whichhas been incorporated herein by reference.

The present invention may use a single active electrode or an electrodearray distributed over a distal contact surface of a probe. Theelectrode array usually includes a plurality of independentlycurrent-limited and/or power-controlled electrode terminals to applyelectrical energy selectively to the target tissue while limiting theunwanted application of electrical energy to the surrounding tissue andenvironment resulting from power dissipation into surroundingelectrically conductive liquids, such as blood, normal saline, and thelike. The electrode terminals may be independently current-limited byusing isolating the terminals from each other and connecting eachterminal to a separate power source that is isolated from the otherelectrode terminals. Alternatively, the electrode terminals may beconnected to each other at either the proximal or distal ends of theprobe to form a single wire that couples to a power source.

The electrosurgical probe will comprise a shaft having a proximal endand a distal end which supports an active electrode. The shaft mayassume a wide variety of configurations, with the primary purpose beingto mechanically support the active electrode and permit the treatingphysician to manipulate the electrode from a proximal end of the shaft.Usually, the shaft will be a narrow-diameter rod or tube, more usuallyhaving dimensions which permit it to be introduced into a body cavity,such as the mouth or the abdominal cavity, through an associated trocaror cannula in a minimally invasive procedure, such as arthroscopic,laparoscopic, thoracoscopic, and other endoscopic procedures. Thus, theshaft will typically have a length of at least 5 cm for oral proceduresand at least 10 cm, more typically being 20 cm, or longer for endoscopicprocedures. The shaft will typically have a diameter of at least 1 mmand frequently in the range from 1 to 10 mm. Of course, fordermatological procedures on the outer skin, the shaft may have anysuitable length and diameter that would facilitate handling by thesurgeon.

The shaft may be rigid or flexible, with flexible shafts optionallybeing combined with a generally rigid external tube for mechanicalsupport. Flexible shafts may be combined with pull wires, shape memoryactuators, and other known mechanisms for effecting selective deflectionof the distal end of the shaft to facilitate positioning of theelectrode array. The shaft will usually include a plurality of wires orother conductive elements running axially therethrough to permitconnection of the electrode array to a connector at the proximal end ofthe shaft. Specific shaft designs will be described in detail inconnection with the figures hereinafter.

The circumscribed area of the electrode array is in the range from 0.25mm² to 75 mm², preferably from 0.5 mm² to 40 mm², and will usuallyinclude at least two isolated electrode terminals, more usually at leastfour electrode terminals, preferably at least six electrode terminals,and often 50 or more electrode terminals, disposed over the distalcontact surfaces on the shaft. By bringing the electrode array(s) on thecontact surface(s) in close proximity with the target tissue andapplying high frequency voltage between the array(s) and an additionalcommon or return electrode in direct or indirect contact with thepatient's body, the target tissue is selectively ablated or cut,permitting selective removal of portions of the target tissue whiledesirably minimizing the depth of necrosis to surrounding tissue. Inparticular, this invention provides a method and apparatus foreffectively ablating and cutting tissue which may be located in closeproximity to other critical organs, vessels or structures (e.g., teeth,bone) by simultaneously (1) causing electrically conducting liquid toflow between the common and active electrodes, (2) applying electricalenergy to the target tissue surrounding and immediately adjacent to thetip of the probe, (3) bringing the active electrode(s) in closeproximity with the target tissue using the probe itself, and (4)optionally moving the electrode array axially and/or transversely overthe tissue.

In one configuration, each individual electrode terminal in theelectrode array is electrically insulated from all other electrodeterminals in the array within said probe and is connected to a powersource which is isolated from each of the other electrodes in the arrayor to circuitry which limits or interrupts current flow to the electrodewhen low resistivity material (e.g., blood or electrically conductivesaline irrigant) causes a lower impedance path between the commonelectrode and the individual electrode terminal. The isolated powersources for each individual electrode may be separate power supplycircuits having internal impedance characteristics which limit power tothe associated electrode terminal when a low impedance return path isencountered, may be a single power source which is connected to each ofthe electrodes through independently actuatable switches or may beprovided by independent current limiting elements, such as inductors,capacitors, resistors and/or combinations thereof. The current limitingelements may be provided in the probe, connectors, cable, controller oralong the conductive path from the controller to the distal tip.Alternatively, the resistance and/or capacitance may occur on thesurface of the active electrode(s) due to oxide layers which formselected electrode terminals (e.g., titanium or a resistive coating onthe surface of metal, such as platinum).

The tip region of the probe may be composed of many independentelectrode terminals designed to deliver electrical energy in thevicinity of the tip. The selective application of electrical energy tothe target tissue is achieved by connecting each individual electrodeterminal and the common electrode to a power source having independentlycontrolled or current limited channels. The common electrode may be atubular member of conductive material proximal to the electrode array atthe tip which also serves as a conduit for the supply of theelectrically conducting liquid between the active and common electrodes.The application of high frequency voltage between the common electrodeand the electrode array results in the generation of high electric fieldintensities at the distal tips of the electrodes with conduction of highfrequently current from each individual electrode terminal to the commonelectrode. The current flow from each individual electrode terminal tothe common electrode is controlled by either active or passive means, ora combination thereof, to deliver electrical energy to the target tissuewhile minimizing energy delivery to surrounding (non-target) tissue andany conductive fluids which may be present (e.g., blood, electrolyticirrigants such as saline, and the like).

In a preferred aspect, this invention takes advantage of the differencesin electrical resistivity between the target tissue (e.g., gingiva,muscle, fascia, tumor, epidermal, heart or other tissue) and thesurrounding conductive liquid (e.g., isotonic saline irrigant). By wayof example, for any selected level of applied voltage, if the electricalconduction path between the common electrode and one of the individualelectrode terminals within the electrode array is isotonic salineirrigant liquid (having a relatively low electrical impedance), thecurrent control means connected to the individual electrode will limitcurrent flow so that the heating of intervening conductive liquid isminimized. On the other hand, if a portion of or all of the electricalconduction path between the common electrode and one of the individualelectrode terminals within the electrode array is gingival tissue(having a relatively higher electrical impedance), the current controlcircuitry or switch connected to the individual electrode will allowcurrent flow sufficient for the deposition of electrical energy andassociated ablation or electrical breakdown of the target tissue in theimmediate vicinity of the electrode surface.

The application of a high frequency voltage between the common or returnelectrode and the electrode array for appropriate time intervals effectsablation, cutting or reshaping of the target tissue. The tissue volumeover which energy is dissipated (i.e., a high voltage gradient exists)may be precisely controlled, for example, by the use of a multiplicityof small electrodes whose effective diameters range from about 2 mm to0.01 mm, preferably from about 1 mm to 0.05 mm, and more preferably fromabout 0.5 mm to 0.1mm. Electrode areas for both circular andnon-circular terminals will have a contact area (per electrode) below 5mm², preferably being in the range from 0.0001 mm² to 1 mm², and morepreferably from 0.005 mm² to 0.5 mm². The use of small diameterelectrode terminals increases the electric field intensity and reducesthe extent or depth of tissue necrosis as a consequence of thedivergence of current flux lines which emanate from the exposed surfaceof each electrode terminal.

Energy deposition in tissue sufficient for irreversible damage (i.e.,necrosis) has been found to be limited to a distance of about one-halfto one electrode diameter. This is a particular advantage over priorelectrosurgical probes employing single and/or larger electrodes wherethe depth of tissue necrosis may not be sufficiently limited.

In previous electrosurgical devices, increased power application andablation rates have been achieved by increasing the electrode area.Surprisingly, with the present invention, it has been found that thetotal electrode area can be increased (to increase power delivery andablation rate) without increasing the depth of necrosis by providingmultiple small electrode terminals. Preferably, the terminals will bespaced-apart by a distance in the range from about one-half diameter toone diameter for optimum power delivery, as discussed below. The depthof necrosis may be further controlled by switching the applied voltageoff and on to produce pulses-of current, the pulses being of sufficientduration and associated energy density to effect ablation and/or cuttingwhile being turned off for periods sufficiently long to allow forthermal relaxation between energy pulses. In this manner, the energypulse duration and magnitude and the time interval between energy pulsesare selected to achieve efficient rates of tissue ablation or cuttingwhile allowing the temperature of the treated zone of tissue to “relax”or return to normal physiologic temperatures (usually to within 10° C.of normal body temperature [37° C.], preferably to within 5° C. beforethe onset of the next energy (current) pulse.

In addition to the above described methods, the applicant has discoveredanother mechanism for ablating tissue while minimizing the depth ofnecrosis. This mechanism involves applying a high frequency voltagebetween the active electrode surface and the return electrode to develophigh electric field intensities in the vicinity of the target tissuesite. The high electric field intensities lead to electric field inducedmolecular breakdown of target tissue through molecular dissociation(rather than thermal evaporation or carbonization). In other words, thetissue structure is volumetrically removed through moleculardisintegration of complex organic molecules into non-viable hydrocarbonsand nitrogen compounds. This molecular disintegration completely removesthe tissue structure, as opposed to transforming the tissue materialfrom a solid form directly to a vapor form, as is typically the casewith ablation.

The high electric field intensities may be generated by applying a highfrequency voltage that is sufficient to vaporize the electricallyconducting liquid over at least a portion of the active electrode(s) inthe region between the distal tip of the active electrode and the targettissue. Since the vapor layer or vaporized region has a relatively highelectrical impedance, it increases the voltages differential between theactive electrode tip and the tissue and causes ionization within thevapor layer due to the presence of an ionizable species (e.g., sodiumwhen isotonic saline is the electrically conducting fluid). Thisionization, under optimal conditions, induces the discharge of energeticelectrons and photons from vapor layer and to the surface of the targettissue. This energy may be in the form of energetic photons (e.g.,ultraviolet radiation), energetic particles (e.g., electrons) or acombination thereof.

The necessary conditions for forming a vapor layer near the activeelectrode tip(s), ionizing the atom or atoms within the vapor layer andinducing the discharge of energy from plasma within the vapor layer willdepend on a variety of factors, such as: the number of electrodeterminals; electrode size and spacing; electrode surface area;asperities and sharp edges on the electrode surfaces; electrodematerials; applied voltage and power; current limiting means, such asinductors; electrical conductivity of the fluid in contact with theelectrodes; density of the fluid; and other factors. Based on initialexperiments, applicants believe that the ionization of atoms within thevapor layer produced in isotonic saline (containing sodium chloride)leads to the generation of energetic photons having wavelengths, by wayof example, in the range of 306 to 315 nanometers (ultraviolet spectrum)and 588 to 590 nanometers (visible spectrum). In addition, the freeelectrons within the ionized vapor layer are accelerated in the highelectric fields near the electrode tip(s). When the density of the vaporlayer (or within a bubble formed in the electrically conducting liquid)becomes sufficiently low (i.e., less than approximately 10²⁰ atoms/cm³for aqueous solutions), the electron mean free path increases to enablesubsequently injected electrons to cause impact ionization within theseregions of low density (i.e., vapor layers or bubbles). Energy evolvedby the energetic electrons (e.g., 4 to 5 eV) can subsequently bombard amolecule and break its bonds, dissociating a molecule into freeradicals, which then combine into final gaseous or liquid species.

The photon energy produces photoablation through photochemical and/orphotothermal processes to disintegrate tissue thicknesses as small asseveral cell layers of tissue at the target site. This photoablation isa “cold” ablation, which means that the photon energy transfers verylittle heat to tissue beyond the boundaries of the region of tissueablated. The cold ablation provided by photon energy can be preciselycontrolled to only affect a thin layer of cells without heating orotherwise damaging surrounding or underlying cells. The depth ofnecrosis will be typically be about 0 to 400 microns and usually 10 to200 microns. Applicants believe that the “fragments” of disintegratedtissues molecules carry away much of the energy which is deposited onthe surface of the target tissue, thereby allowing moleculardisintegration of tissue to occur while limiting the amount of heattransfer to the surrounding tissue.

In addition, other competing mechanisms may be contributing to theablation of tissue. For example, tissue destruction or ablation may alsobe caused by dielectric breakdown of the tissue structural elements orcell membranes from the highly concentrated intense electric fields atthe tip portions of the electrode(s). According to the teachings of thepresent invention, the active electrode(s) are sized and have exposedsurfaces areas which, under proper conditions of applied voltage, causethe formation of a vaporized region or layer over at least a portion ofthe surface of the active electrode(s). This layer or region ofvaporized electrically conducting liquid creates the conditionsnecessary for ionization within the vaporized region or layer and thegeneration of energetic electrons and photons. In addition, this layeror region of vaporized electrically conducting liquid provides a highelectrical impedance between the electrode and the adjacent tissue sothat only low levels of current flow across the vaporized layer orregion into the tissue, thereby minimizing joulean heating in, andassociated necrosis of, the tissue.

As discussed above, applicants have found that the density of theelectrically conducting liquid at the distal tips of the activeelectrodes should be less than a critical value to form a suitable vaporlayer. For aqueous solutions, such as water or isotonic saline, thisupper density limit is approximately 10²⁰ atoms/cm³, which correspondsto about 3×10⁻³ grams/cm³. Applicant's also believe that once thedensity in the vapor layer reaches a critical value (e.g., approximately10²⁰ atoms/cm³ for aqueous solutions), electron avalanche occurs. Thegrowth of this avalanche is retarded when the space charge generatedfields are on the order of the external field. Spatial extent of thisregion should be larger than the distance required for an electronavalanche to become critical and for an ionization front to develop.This ionization front develops and propagates across the vapor layer viaa sequence of processes occurring the region ahead of the front, viz,heat by electron injection, lowering of the local liquid density belowthe critical value and avalanche growth of the charged particleconcentration.

Electrons accelerated in the electric field within the vapor layer willapparently become trapped after one or a few scatterings. These injectedelectrons serve to create or sustain a low density region with a largemean free path to enable subsequently injected electrons to cause impactionization within these regions of low density. The energy evolved ateach recombination is on the order of half of the energy band gap (i.e.,4 to 5 eV). It appears that this energy can be transferred to anotherelectron to generate a highly energetic electron. This second, highlyenergetic electron may have sufficient energy to bombard a molecule tobreak its bonds, i.e., dissociate the molecule into free radicals.

The electrically conducting liquid should have a threshold conductivityin order to suitably ionize the vapor layer for the inducement ofenergetic electrons and photons. The electrical conductivity of thefluid (in units of milliSiemans per centimeter or mS/cm) will usually begreater than 0.2 mS/cm, preferably will be greater than 2 mS/cm and morepreferably greater than 10 mS/cm. In an exemplary embodiment, theelectrically conductive fluid is isotonic saline, which has aconductivity of about 17 mS/cm. The electrical conductivity of thechannel trailing the ionization front should be sufficiently high tomaintain the energy flow required to heat the liquid at the ionizationfront and maintain its density below the critical level. In addition,when the electrical conductivity of the liquid is sufficiently high,ionic pre-breakdown current levels (i.e., current levels prior to theinitiation of ionization within the vapor layer) are sufficient to alsopromote the initial growth of bubbles within the electrically conductingliquid (i.e., regions whose density is less than the critical density).

Asperities on the surface of the active electrode(s) appear to promotelocalized high current densities which, in turn, promote bubblenucleation at the site of the asperities whose enclosed density (i.e.,vapor density) is below the critical density to initiate ionizationbreakdown within the bubble. Hence, a specific configuration of thepresent invention creates regions of high current densities on the tipsof the electrode(s) (i.e., the surface of the electrode(s) which are toengage and ablate or cut tissue). Regions of high current densities canbe achieved via a variety of methods, such as producing sharp edges andcorners on the distal tips of the electrodes or vapor blasting,chemically etching or mechanically abrading the distal end faces of theactive electrodes to produce surface asperities thereon. Alternatively,the electrode terminals may be specifically designed to increase theedge/surface area ratio of the electrode terminals. For example, theelectrode terminal(s) may be hollow tubes having a distal,circumferential edge surrounding an opening. The terminals may be formedin an array as described above or in a series of concentric terminals onthe distal end of the probe. High current densities will be generatedaround the circumferential edges of the electrode terminals to promotenucleate bubble formation.

The voltage applied between the common electrode and the electrode arraywill be at high or radio frequency, typically between about 5 kHz and 20MHz, usually being between about 30 kHz and 2.5 MHz, and preferablybeing between about 50 kHz and 400 kHz. The RMS (root mean square)voltage applied will usually be in the range from about 5 volts to 1000volts, preferably being in the range from about 50 volts to 800 volts,and more preferably being in the range from about 100 volts to 400volts. These frequencies and voltages will result in peak-to-peakvoltages and current that are sufficient to vaporize the electricallyconductive liquid and, in turn, create the conditions within thevaporized region which result in high electric fields and emission ofenergetic photons and/or electrons to ablate tissue. Typically, thepeak-to-peak voltage will be in the range of 200 to 2000 volts andpreferably in the range of 300 to 1400 volts and more preferably in therange of 700 to 900 volts.

As discussed above, the voltage is usually delivered in a series ofvoltage pulses with a sufficiently high frequency (e.g., on the order of5 kHz to 20 MHz) such that the voltage is effectively appliedcontinuously (as compared with e.g., lasers claiming small depths ofnecrosis, which are generally pulsed about 10 to 20 Hz). In addition,the pulsed laser duty cycle (i.e., cumulative time in any one-secondinterval that energy is applied) is on the order of about 50% for thepresent invention, as compared with lasers which typically have a dutycycle of about 0.0001%.

Applicants believe that the present invention is capable of obtaininghigh ablation rates with effectively continuous mode operation and highduty cycles because the source of energy emitted from the edges and tipsof the small electrode terminals is effectively a point source or asource having a relatively small effective radius. As is well known inthe art, the flux emitted from a point source and crossing a boundary inspherical space generally decreases as the square of distance from thesource. Thus, the “energy source” of the present invention (i.e., theintense electric field, the energetic photons or the energeticelectrons) is highly concentrated by virtue of the geometry of theemitting electrodes and the source of energy at the tips of theelectrodes. As a result, only those regions or areas that are very closeto the electrode tips or source will be exposed to high energy fluxes.Consequently, ablation will typically only occur in tissue layerseffectively in contact or in very close proximity with the tips of theelectrodes. The tissue at greater distances from the electrode tips arenot significantly affected since the energy flux is too low at thesedistances to irreversibly affect or damage tissue.

Usually, the current level will be selectively limited or controlled andthe voltage applied will be independently adjustable, frequently inresponse to the resistance of tissues and/or fluids in the pathwaybetween an individual electrode and the common electrode. Also, theapplied current level may be in response to a temperature control meanswhich maintains the target tissue temperature with desired limits at theinterface between the electrode arrays and the target tissue. Thedesired tissue temperature along a propagating surface just beyond theregion of ablation will usually be in the range from about 40° C. to100° C. , and more usually from about 50° C. to 60° C. The tissue beingablated (and hence removed from the operation site) immediately adjacentthe electrode array may reach even higher temperatures.

The preferred power source of the present invention delivers a highfrequency current selectable to generate average power levels rangingfrom tens of milliwatts to tens of watts per electrode, depending on thetarget tissue being ablated, the rate of ablation desired or the maximumallowed temperature selected for the probe tip. The power source allowsthe user to select the current level according to the specificrequirements of a particular oral surgery, dermatological procedure,open surgery or other endoscopic surgery procedure.

The power source may be current limited or otherwise controlled so thatundesired heating of electrically conductive fluids or other lowelectrical resistance media does not occur. In a presently preferredembodiment of the present invention, current limiting inductors areplaced in series with each independent electrode terminal, where theinductance of the inductor is in the range of 10 uH to 50,000 uH,depending on the electrical properties of the target tissue, the desiredablation rate and the operating frequency. Alternatively,capacitor-inductor (LC) circuit structures may be employed, as describedpreviously in co-pending PCT application No. PCT/US94/05168, thecomplete disclosure of which is incorporated herein by reference.Additionally, current limiting resistors may be selected. Preferably,these resistors will have a large positive temperature coefficient ofresistance so that, as the current level begins to rise for anyindividual electrode in contact with a low resistance medium (e.g.,saline irrigant), the resistance of the current limiting resistorincreases significantly, thereby minimizing the power delivery from saidelectrode into the low resistance medium (e.g., saline irrigant).

As an alternative to such passive circuit structures, regulated currentflow to each electrode terminal may be provided by a multi-channel powersupply. A substantially constant current level for each individualelectrode terminal within a range which will limit power deliverythrough a low resistance path, e.g., isotonic saline irrigant, and wouldbe selected by the user to achieve the desired rate of cutting orablation. Such a multi-channel power supply thus provides asubstantially constant current source with selectable current level inseries with each electrode terminal, wherein all electrodes will operateat or below the same, user selectable maximum current level. Currentflow to all electrode terminals could be periodically sensed and stoppedif the temperature measured at the surface of the electrode arrayexceeds user selected limits. Particular control system designs forimplementing this strategy are well within the skill of the art.

Yet another alternative involves the use of one or several powersupplies which allow one or several electrodes to be simultaneouslyenergized and which include active control means for limiting currentlevels below a preselected maximum level. In this arrangement, only oneor several electrodes would be simultaneously energized for a briefperiod. Switching means would allow the next one or several electrodesto be energized for a brief period. By sequentially energizing one orseveral electrodes, the interaction between adjacent electrodes can beminimized (for the case of energizing several electrode positioned atthe maximum possible spacing within the overall envelope of theelectrode array) or eliminated (for the case of energizing only a singleelectrode at any one time). As before, a resistance measurement meansmay be employed for each electrode prior to the application of powerwherein a (measured) low resistance (below some preselected level) willprevent that electrodes from being energized during a given cycle. Byway of example, the sequential powering and control scheme of thepresent invention would function in a manner similar to an automobiledistributor. In this example, an electrical contact rotates pastterminals connected to each spark plug. In this example, each spark plugcorresponds to the exposed surface of each of the electrodes. Inaddition, the present invention includes the means to measure theresistance of the medium in contact with each electrode and causevoltage to be applied only if the resistance exceeds a preselectedlevel.

It should be clearly understood that the invention is not limited toelectrically isolated electrode terminals, or even to a plurality ofelectrode terminals. For example, the array of active electrodeterminals may be connected to a single lead that extends through theprobe shaft to a power source of high frequency current. Alternatively,the probe may incorporate a single electrode that extends directlythrough the probe shaft or is connected to a single lead that extends tothe power source.

The active electrode(s) are formed over a contact surface on the shaftof the electrosurgical probe. The common (return) electrode surface willbe recessed relative to the distal end of the probe and may be recessedwithin the conduit provided for the introduction of electricallyconducting liquid to the site of the target tissue and activeelectrode(s). In the exemplary embodiment, the shaft will be cylindricalover most of its length, with the contact surface being formed at thedistal end of the shaft. In the case of endoscopic applications, thecontact surface may be recessed since it helps protect and shield theelectrode terminals on the surface while they are being introduced,particularly while being introduced through the working channel of atrocar channel or a viewing scope.

The area of the contact surface can vary widely, and the contact surfacecan assume a variety of geometries, with particular areas in geometriesbeing selected for specific applications. Active electrode contactsurfaces can have areas in the range from 0.25 mm² to 50 mm², usuallybeing from 1 mm² to 20 mm². The geometries can be planar, concave,convex, hemispherical, conical, linear “in-line” array or virtually anyother regular or irregular shape. Most commonly, the active electrode(s)will be formed at the distal tip of the electrosurgical probe shaft,frequently being planar, disk-shaped, or hemispherical surfaces for usein reshaping procedures or being linear arrays for use in cutting.Alternatively or additionally, the active electrode(s) may be formed onlateral surfaces of the electrosurgical probe shaft (e.g., in the mannerof a spatula), facilitating access to certain body structures inelectrosurgical procedures.

During the surgical procedure, the distal end of the probe or the activeelectrode(s) will be maintained at a small distance away from the targettissue surface. This small spacing allows for the continual resupply ofelectrically conducting liquid into the interface between the activeelectrode(s) and the target tissue surface. This continual resupply ofthe electrically conducting liquid helps to ensure that the thin vaporlayer will remain between active electrode(s) and the tissue surface. Inaddition, dynamic movement of the active electrode(s) over the tissuesite allows the electrically conducting liquid to cool the tissuesurrounding recently ablated areas to minimize thermal damage to thissurrounding tissue. Typically, the active electrode(s) will be about0.02 to 2 mm from the target tissue and preferably about 0.05 to 0.5 mmduring the ablation process. One method of maintaining this space is totranslate and/or rotate the probe transversely relative to the tissue,i.e., a light brushing motion, to maintain a thin vaporized layer orregion between the active electrode and the tissue.35 Of course, ifcoagulation of a deeper region of tissue is necessary (e.g., for sealinga bleeding vessel imbedded within the tissue), it may be desirable topress the active electrode against the tissue to effect joulean heatingtherein.

Referring to the drawings in detail, wherein like numerals indicate likeelements, an electrosurgical system 11 is shown constructed according tothe principles of the present invention. Electrosurgical system 11generally comprises an electrosurgical probe 10 connected to a powersupply 23 for providing high frequency voltage to a target tissue 52 anda liquid source 21 for supplying electrically conducting fluid 50 toprobe 10.

In an exemplary embodiment as shown in FIG. 1, electrosurgical probe 10includes an elongated shaft 13 which may be flexible or rigid, withflexible shafts optionally including support cannulas or otherstructures (not shown). Probe 10 includes a connector 19 at its proximalend and an array 12 of electrode terminals 58 disposed on the distal tipof shaft 13. A connecting cable 34 has a handle 22 with a connector 20which can be removably connected to connector 19 of probe 10. The.proximal portion of cable 34 has a connector 26 to couple probe 10 topower supply 28. The electrode terminals 58 are electrically isolatedfrom each other and each of the terminals 58 is connected to an activeor passive control network within power supply 28 by means of aplurality of individually insulated conductors 42 (see FIG. 2C). Powersupply 28 has a selection means 30 to change the applied voltage level.Power supply 28 also includes means for energizing the electrodes 58 ofprobe 10 through the depression of a pedal 39 in a foot pedal 37positioned close to the user. The foot pedal 37 may also include asecond pedal (not shown) for remotely adjusting the energy level appliedto electrodes 58. The specific design of a power supply which may beused with the electrosurgical probe of the present invention isdescribed in parent application PCT US Ser. No. 94/051168, the fulldisclosure of which has previously been incorporated herein byreference.

Referring to FIGS. 2A and 2B, the electrically isolated electrodeterminals 58 are spaced-apart over an electrode array surface 82. Theelectrode array surface 82 and individual electrode terminals 58 willusually have dimensions within the ranges set forth above. In thepreferred embodiment, the electrode array surface 82 has a circularcross-sectional shape with a diameter D (FIG. 2B) in the range from 0.3mm to 10 mm. Electrode array surface 82 may also have an oval shape,having a length L in the range of 1 mm to 20 mm and a width W in therange from 0.3 mm to 7 mm, as shown in FIG. 5. The individual electrodeterminals 58 will protrude over the electrode array surface 82 by adistance (H) from 0 mm to 2 mm, preferably from 0 mm to 1 mm (see FIG.3).

It should be noted that the electrode terminals may be flush with theelectrode array surface 82, or the terminals may be recessed from thesurface. For example, in dermatological procedures, the electrodeterminals 58 may be recessed by a distance from 0.01 mm to 1 mm,preferably 0.01 mm to 0.2 mm. In one embodiment of the invention, theelectrode terminals are axially adjustable relative to the electrodearray surface 82 so that the surgeon can adjust the distance between thesurface and the electrode terminals.

The electrode terminals 58 are preferably composed of a refractory,electrically conductive metal or alloy, such. as platinum, titanium,tantalum, tungsten and the like. As shown in FIG. 2B, the electrodeterminals 58 are anchored in a support matrix 48 of suitable insulatingmaterial (e.g., ceramic or glass material, such as alumina, zirconia andthe like) which could be formed at the time of manufacture in a flat,hemispherical or other shape according to the requirements of aparticular procedure. The preferred support matrix material is alumina,available from Kyocera Industrial Ceramics Corporation, Elkgrove, Ill.,because of its high thermal conductivity, good electrically insulativeproperties, high flexural modulus, resistance to carbon tracking,biocompatibility, and high melting point.

As shown in FIG. 2A, the support matrix 48 is adhesively joined to atubular support member 78 that extends most or all of the distancebetween matrix 48 and the proximal end of probe 10. Tubular member 78preferably comprises an electrically insulating material, such as anepoxy, injection moldable plastic or silicone-based material. In apreferred construction technique, electrode terminals 58 extend throughpre-formed openings in the support matrix 48 so that they protrude aboveelectrode array surface 82 by the desired distance H (FIG. 3). Theelectrodes may then be bonded to the distal surface 82 of support matrix48, typically by an inorganic sealing material 80. Sealing material 80is selected to provide effective electrical insulation, and goodadhesion to both the ceramic matrix 48 and the platinum or titaniumelectrode terminals. Sealing material 80 additionally should have acompatible thermal expansion coefficient and a melting point well belowthat of platinum or titanium and alumina or zirconia, typically being aglass or glass ceramic.

In the embodiment shown in FIGS. 2A and 2B, probe 10 includes a returnelectrode 56 for completing the current path between electrode terminals58 and power supply 28. Return electrode 56 is preferably an annularmember positioned around the exterior of shaft 13 of probe 10. Returnelectrode 56 may fully or partially circumscribe tubular support member78 to form an annular gap 54 therebetween for flow of electrically.conducting liquid 50 therethrough, as discussed below. Gap 54 preferablyhas a width in the range of 0.15 mm to 4 mm. Return electrode 56 extendsfrom the proximal end of probe 10, where it is suitably connected topower supply 28 via connectors 19, 20, to a point slightly proximal ofelectrode array surface 82, typically about 0.5 to 10 mm and morepreferably about 1 to 10 mm.

Return electrode 56 is disposed within an electrically insulative jacket18, which is typically formed as one or more electrically insulativesheaths or coatings, such as polytetrafluoroethylene, polyimide, and thelike. The provision of the electrically insulative jacket 18 over returnelectrode 56 prevents direct electrical contact between return electrode56 and any adjacent body structure or the surgeon. Such directelectrical contact between a body structure (e.g., tendon) and anexposed common electrode member 56 could result in unwanted heating andnecrosis of the structure at the point of contact causing necrosis.

Return electrode 56 is preferably formed from an electrically conductivematerial, usually metal, which is selected from the group consisting ofstainless steel alloys, platinum or its alloys, titanium or its alloys,molybdenum or its alloys, and nickel or its alloys. The return electrode56 may be composed of the same metal or alloy which forms the electrodeterminals 58 to minimize any potential for corrosion or the generationof electrochemical potentials due to the presence of dissimilar metalscontained within an electrically conductive fluid 50, such as isotonicsaline (discussed in greater detail below).

As shown in FIG. 2A, return electrode 56 is not directly connected toelectrode terminals 58. To complete this current path so that terminals58 are electrically connected to return electrode 56 via target tissue52, electrically conducting liquid 50 (e.g., isotonic saline) is causedto flow along liquid paths 83. A liquid path 83 is formed by annular gap54 between outer return electrode 56 and tubular support member 78. Anadditional liquid path 83 may be formed between an inner lumen 57 withinan inner tubular member 59. However, it is generally preferred to formthe liquid path 83 near the perimeter of the probe so that theelectrically conducting liquid tends to flow radially inward towards thetarget site 88 (this preferred embodiment is illustrated in FIGS. 8-19).In the embodiment shown in FIGS. 2-5, the liquid flowing through innerlumen 57 may tend to splash radially outward, drawing electrical currenttherewith and potentially causing damage to the surrounding tissue.

The electrically conducting liquid 50 flowing through fluid paths 83provides a pathway for electrical current flow between target tissue 52and return electrode 56, as illustrated by the current flux lines 60 inFIG. 2A. When a voltage difference is applied between electrode array 12and return electrode 56, high electric field intensities will begenerated at the distal tips of terminals 58 with current flow fromarray 12 through the target tissue to the return electrode, the highelectric field intensities causing ablation of tissue 52 in zone 88.

FIGS. 2C, 3 and 4 illustrate an alternative embodiment ofelectrosurgical probe 10 which has a return electrode 55 is positionedwithin tubular member 78. Return electrode 55 is preferably a tubularmember defining an inner lumen 57 for allowing electrically conductingliquid 50 (e.g., isotonic saline) to flow therethrough in electricalcontact with return electrode 55. In this embodiment, a voltagedifference is applied between electrode terminals 58 and returnelectrode 55 resulting in electrical current flow through theelectrically conducting liquid 50 as shown by current flux lines 60(FIG. 3). As a result of the applied voltage difference and concomitanthigh electric field intensities at the tips of electrode terminals 58,tissue 52 becomes ablated or transected in zone 88.

FIG. 2C illustrates the proximal or connector end 70 of probe 10 in theembodiment of FIGS. 3 and 4. Connector 19 comprises a plurality ofindividual connector pins 74 positioned within a housing 72 at theproximal end 70 of probe 10. Electrode terminals 58 and the attachedinsulating conductors 42 extend proximally to connector pins 74 inconnector housing 72. Return electrode 55 extends into housing 72, whereit bends radially outward to exit probe 10. As shown in FIGS. 1 and 2C,a liquid supply tube 15 removably couples liquid source 21, (e.g., a bagof fluid elevated above the surgical site or having a pumping device),with return electrode 55. Preferably, an insulating jacket 14 covers theexposed portions of electrode 55. One of the connector pins 76 iselectrically connected to return electrode 55 to couple electrode 55 topower supply 28 via cable 34. A manual control valve 17 may also beprovided between the proximal end of electrode 55 and supply tube 15 toallow the surgical team to regulate the flow of electrically conductingliquid 50.

FIG. 6 illustrates another embodiment of probe 10 where the distalportion of shaft 13 is bent so that electrode terminals extendtransversely to the shaft. Preferably, the distal portion of shaft 13 isperpendicular to the rest of the shaft so that electrode array surface82 is generally parallel to the shaft axis, as shown in FIG. 6. In thisembodiment, return electrode 55 is mounted to the outer surface of shaft13 and is covered with an electrically insulating jacket 18. Theelectrically conducting fluid 50 flows along flow path 93 through returnelectrode 55 and exits the distal end of electrode 55 at a pointproximal of electrode surface 82. The fluid is directed exterior ofshaft to electrode surface 82 to create a return current path fromelectrode terminals 58, through target tissue 52, to return electrode55, as shown by current flux lines 60.

FIG. 7 illustrates another embodiment of the invention whereelectrosurgical system 11 further Includes a liquid supply instrument 64for supplying electrically conducting fluid 50 between electrodeterminals 58 and return electrode 55. Liquid supply instrument 64comprises an inner tubular member or return electrode 55 surrounded byan electrically insulating jacket 18. Return electrode 55 defines aninner passage 83 for flow of fluid 50. As shown in FIG. 7, the distalportion of instrument 64 is preferably bent so that liquid 50 isdischarged at an angle with respect to instrument 64. This allows thesurgical team to position liquid supply instrument 64 adjacent electrodesurface 82 with the proximal portion of supply instrument 64 oriented ata similar angle to probe 10.

FIGS. 8 and 9 illustrate another embodiment of probe where the returnelectrode is an outer tubular member 56 that circumscribes supportmember 78 and conductors 42. Insulating jacket 18 surrounds tubularmember 56 and is spaced from member 56 by a plurality of longitudinalribs 96 to define an annular gap 54 therebetween (FIG. 9). Annular gappreferably has a width in the range of 0.15 mm to 4 mm. Ribs 96 can beformed on either the jacket 18 or member 56. The distal end of returnelectrode 56 is a distance L₁ from electrode support surface 82.Distance L₁ is preferably about 0.5 to 10 mm and more preferably about 1to 10 mm. The length L₁ of return electrode 56 will generally depend onthe electrical conductivity of the irrigant solution.

As shown in FIG. 8, electrically conducting liquid flows through annulargap 54 (in electrical communication with the return electrode) and isdischarged through the distal end of gap 54. The liquid 50 is thendirected around support member 78 to electrode terminals 58 to providethe current pathway between the electrode terminals and return electrode56. Since return electrode 56 is proximally recessed with respect toelectrode surface 82, contact between the return electrode 56 andsurrounding tissue is minimized. In addition, the distance L₁ betweenthe active electrode terminals 58 and the return electrode 56 reducesthe risk of current shorting therebetween.

The present invention is not limited to an electrode array disposed on arelatively planar surface at the distal tip of probe 10, as describedabove. Referring to FIGS. 12∝14, an alternative probe 10 includes a pairof electrodes 58 a, 58 b mounted to the distal end of shaft 13.Electrodes 58 a, 58 b are electrically connected to power supply asdescribed above and preferably have tips 100 a, 100 b with a screwdriveror flattened shape. The screwdriver shape provides a greater amount of“edges” to electrodes 58 a, 58 b, to increase the electric fieldintensity and current density at the edges and thereby improve thecutting ability as well as the ability to limit bleeding from theincised tissue (i.e., hemostasis).

As shown in FIG. 12, current flows between electrode tips 100 a and 100b as indicated by current flux lines 60 to heat the target tissue 52.The surgeon then moves probe 10 transversely across tissue 52 to effectan incision 102 in tissue 52, as shown in FIG. 14.

Other modifications and variations can be made to disclose embodimentswithout departing from the subject invention as defined in the followingclaims. For example, shaft 13 of probe 10 may have a variety ofconfigurations other than the generally linear shape shown in FIGS. 1-8.For example, shaft 13 may have a distal portion that is angled, in therange of 10° to 30° (FIG. 10) or 90° (FIGS. 11 and 6), to improve accessto the operative site of the tissue 52 being ablated or cut (see FIG.10). A shaft having a 90° bend angle may be particular useful foraccessing gingiva located in the back portion of the patient's mouth anda shaft having a 10° to 30° bend angle may be useful for accessinggingiva near or in the front of the patient's mouth.

In addition, it should be noted that the invention is not limited to anelectrode array comprising a plurality of active electrodes. Theinvention could utilize a plurality of return electrodes, e.g., in abipolar array or the like. In addition, depending on other conditions,such as the peak-to-peak voltage, electrode diameter, etc., a singleactive electrode may be sufficient to develop a vapor layer and inducethe discharge of energy to ablate or cut tissue, as described above.

By way of example, FIGS. 21 and 22 illustrate the design of a probe 10according to the present invention comprising a single active electrode58 having a tubular geometry. As described above, the return electrodemay be an outer tubular member 56 that circumscribes insulated conductor42 and adhesive bonding material 79 which, in turn, adhesively joins toactive electrode support members 48 a and 48 b.

Electrode support members 48 a and 48 b may be ceramic, glass ceramic orother electrically insulating material which resists carbon or arctracking. A preferred electrode support member material is alumina. Inthe example embodiment, a solid rod of alumina forms an inner portion 48b of electrode support member 48 and a hollow tube of alumina forms anouter portion 48 a of electrode support member 48. Tubular shaped activeelectrode 58 may be fabricated using shaped cylinder of this metalcomprising an electrically conductive metal, such as platinum, tantalum,tungsten, molybdenum, columbium or alloys thereof. Active electrode 58is connected to connector 19 (see FIG. 2C) via an insulated lead 108. Anelectrically insulating jacket 18 surrounds tubular member 56 and may bespaced from member 56 by a plurality of longitudinal ribs 96 to definean annular gap 54 therebetween (FIG. 22). Annular gap 54 preferably hasa width in the range of 0.15 to 4 mm. Ribs 96 can be formed on eitherjacket 18 or tubular member 56. The distal end of the return electrode56 is a distance L₁ from electrode support surface 82. Distance L₁ ispreferably about 0.5 mm to 10 mm and more preferably about 1 to 10 mm.The length L₁ of return electrode 56 will generally depend on theelectrical conductivity of the irrigant solution.

As shown in FIG. 21, electrically conducting liquid 50 flows throughannular gap 54 (in electrical communication with return electrode 56)and is discharged through the distal end of gap 54. The liquid 50 isthen directed around electrode support member 48 a to electrode terminal58 to provide he current pathway between electrode terminal 58 andreturn electrode 56. As described above, the active and returnelectrodes are connected to voltage supply 28 via cable 34 (see FIG. 1).

FIGS. 23 and 24 illustrate further embodiments of electrosurgical probesaccording to the present invention. In FIG. 23, a probe 10 comprises amultiplicity of electrodes 58 which converge to a single electrode lead42. As shown, a central electrode 105 extends to the proximal end of theprobe shaft for connection to connector 19 (FIG. 2C). The remainder ofthe electrodes 58 extend through a portion of the probe shaft and areelectrically coupled to central electrode 105 by, for example, a weld,solder joint or crimp connection 100. In FIG. 24, an electrosurgicalprobe 10 comprises a single electrode 58 connected to a single electrodelead 42. As described above, the active and return electrodes areconnected to voltage supply 28 via cable 34 (see FIG. 1).

Both of the single active electrode configurations depicted in FIGS.21-24 may be used with the integral supply means and return electrodesdescribed above in FIGS. 2-11, 30 and 31. Alternatively, these probeconfigurations may be operated in body cavities already containing anelectrically conducting liquid 50, obviating the need for either anintegral supply of said liquid or an electrically insulating sleeve toform a conduit for supply of the electrically conducting liquid 50.Instead, an electrically insulating covering would be applied tosubstantially all of the return electrode 56 (other than the proximalportion).

FIG. 15 illustrates the current flux lines associated with an electricfield 120 applied between the active and return electrodes 56, 58 when avoltage is applied therebetween. As shown, the electric field intensityis substantially higher in the region 88 at the tip of the electrode 56because the current flux lines are concentrated in these regions. Thishigh electric field intensity leads to induced molecular breakdown ofthe target tissue through molecular dissociation. Preferably, theelectric field intensity is sufficient to ionize the vaporizedelectrically conducting liquid 50 in a thin layer 124 between the distaltip 122 of the active electrode 58 and the target tissue 52, as shown inFIG. 16. The vapor layer 124 will usually have a thickness of about 0.02to 2.0 mm.

As shown in FIG. 16, the electric field ionizes the vapor layer due tothe presence of an ionizable species (e.g., sodium) within the vaporlayer to create a plasma. This ionization, under optimal conditions,induces the discharge of highly energetic electrons and/or photons fromthe vapor layer. The photon and/or the energetic electrons causedisintegration of the tissue molecules adjacent to the vapor layer. FIG.16 illustrates the issuance of bubbles 126 of non-condensible gaseousproducts resulting from the disintegration of tissue at the target site.

The system and method of the present invention is also useful indermatological procedures, i.e., surface tissue ablation on thepatient's outer skin or epidermis. For example, the probe of the presentinvention can be used for the removal of tissue abnormalities,pigmentations, such as freckles, tattoos, age or liver spots, birthmarks, malignant melanomas, and-superficial lentigines in the epidermis,and other unwanted tissue, such as soft fatty tissue, cutaneousangiodysplasia, e.g., skin angloma, malignant tumor tissue, lumbago(i.e., tissue bulges extending from the vertebrae) or the like. Inaddition, the probe of the present invention may be used for removingsurface layers of the epidermis to provide younger looking skin (tissuerejuvenation) or for incising, dividing and resecting tissue duringcosmetic surgery procedures.

FIG. 17 illustrates an exemplary embodiment, where an electrosurgicalprobe 130 is utilized to remove the surface layers of the epidemic 140.Probe 120 includes a shaft 132 coupled to a proximal handle 134 forholding and controlling shaft 132. Similar to previous embodiments,probe 130 includes an active electrode array 136 at the distal tip ofshaft 132, an annular return electrode 138 extending through shaft 132and proximally recessed from the active electrode array 136 and anannular lumen 142 between return electrode 138 and an outer insulatingsheath 144. Probe 130 further includes a liquid supply conduit 146attached to handle 134 and in fluid communication with lumen 142 and asource of electrically conducting fluid (not shown) for delivering thefluid past return electrode 138 to the target site on the is epidermis140. As discussed above, electrode array 136 is preferably flush withthe distal end of shaft 132 or distally extended from the distal end bya small distance (on the order of 0.005 inches) so to minimize the depthof ablation. Preferably, the distal end of shaft 132 is beveled toimprove access and control of probe 130 while treating the epidermaltissue.

The voltage will preferably be sufficient to establish high electricfield intensities between the active electrode array 136 and theepidermal tissue 140 to thereby induce molecular breakdown ordisintegration of several cell layers of the epidermal tissue. Asdescribed above, a sufficient voltage will be applied to develop a thinlayer of vapor within the electrically conducting fluid and to ionizethe vaporized layer or region between the active electrode(s) and thetarget tissue. Energy in the form of photons and/or energetic electronsare discharged from the vapor layer to ablate the epidermal tissue,thereby minimizing necrosis of surrounding tissue and underlying celllayers, such as cell structures in the stratum lucidium and/or stratumgranulosum.

FIGS. 18-20 illustrate an exemplary embodiment of another importantapplication of the present invention. As discussed above, the probe ofthe present invention may be particularly useful for boring a channelthrough tissue by axially translating the probe towards the tissue asthe tissue is disintegrated by the mechanisms discussed above. In theexemplary embodiment, the probe of the present invention is used in atransmyocardial revascularization procedure to form channels from themyocardium to the ventricular cavity to perfuse the myocardium. Thisprocedure is an alternative to coronary artery bypass surgery fortreating coronary artery disease. The channels allow oxygen enrichedblood flowing into the ventricular cavity from the aorta to directlyflow into the myocardium; rather than exiting the heart and then flowingback into the myocardium through the coronary arteries.

As shown in FIG. 18, electrosurgical probe 10 is positioned into one ofthe ventricular cavities of the heart, in this case, the right ventricle200. Electrosurgical probe 10 may be introduced into the right ventricle200 in a variety of procedures that are well known in the art, such as athoracotomy, sternotomy or minimally invasive procedures. In therepresentative embodiment, probe 10 is introduced into the vasculatureof the patient through a percutaneous penetration and axially translatedvia a guide catheter 202 through one of the major vessels to the rightventricular cavity 204. A preferred embodiment incorporates a steerableguide catheter 202 which can be externally controlled by the surgeon todirect the distal portion of the guide catheter 202 and probe 10 to thetarget site(s) in ventricular cavity 204.

Referring to FIG. 19, ventricle wall 206 comprises an epicardium 208, amyocardium 210 and an endocardium 212. In the representative embodiment,probe 10 will form a channel 214 or artificial vessel from theventricular cavity 206, through the endocardium 212 and into themyocardium 210 to thereby increase myocardial blood flow from theendocardium 212 to the myocardium 210. The location of channel 214 maybe selected based on familiar epicardial anatomic landmarks, such as theepicardial branches of the coronary arteries. Guide catheter 202 ispositioned adjacent the inner endocardial wall and probe 10 is axiallytranslated so that the active electrode 58 at its distal end ispositioned proximate the heart tissue. In this embodiment, the probeincludes a single, annular electrode 58 at its distal tip for ablationof the heart tissue. However, it will be readily recognized that theprobe may include an array or electrode terminals as described in detailabove.

Electrically conducting liquid 50 is delivered through an annular lumen220 between an annular return electrode 222 and an insulating sheath 224of the probe. Return electrode 222 is recessed from the distal end ofactive electrode 58, preferably about 0.025 to 0.050 inches.Alternatively, the return electrode may be positioned on the exteriorsurface (skin) of the patient, or it may be located nearby on a moreproximal position of the probe. Similar to the above embodiments, a highfrequency voltage (e.g., 100 kHz) is applied between active electrode(s)58 and return electrode 222 to establish a current flow therebetweenthat ablates or disintegrates the heart tissue. The high frequencyvoltage will preferably be sufficient to vaporize a thin layer of theelectrically conducting liquid and to induce the discharge of photonand/or electron energy from the vapor layer to provide cold ablation ofthe heart tissue.

Ablation of the tissue may be facilitated by axially reciprocatingand/or rotating the probe within guide catheter 202 a distance ofbetween about 0.05 to 0.20 inches. This axial reciprocation or rotationallows the electrically conducting liquid 50 to flow over the tissuesurface being canalized, thereby cooling this tissue and preventingsignificant thermal damage to the surrounding tissue cells.

FIG. 20 illustrates an alternative embodiment of the probe of FIG. 1. Inthis embodiment, the probe 260 includes a central lumen 262 having aproximal end attached to a suitable vacuum source (not shown) and anopen distal end 266 for aspirating the target site. The active electrodeis preferably a single annular electrode 268 surrounding the open distalend 266 of central lumen 262. Central lumen 262 is utilized to removethe ablation products (e.g., liquids and gases) generated at the targetsite and excess electrically conductive irrigant during the procedure.

In both of the above embodiments, the present invention provideslocalized ablation or disintegration of heart tissue to form arevascularization channel 214 of controlled diameter and depth. Usually,the diameter will be in the range of 0.5 mm to 3 mm. Preferably, theradio frequency voltage will be in the range of 400 to 1400 voltspeak-to-peak to provide controlled rates of tissue ablation andhemostasis while minimizing the depth of necrosis of tissue surroundingthe desired channel. This voltage will typically be applied continuouslythroughout the procedure until the desired length of the channel 214 iscompletely formed. However, the heartbeat may be monitored and thevoltage applied in pulses that are suitably timed with the contractions(systole) of the heart.

It should be noted that the above embodiment is merely representativeand is not intended to limit the invention. For example, theelectrosurgical probe can be used to effect a myocardialrevascularization channel from the exterior of the heart into theventricular cavity. In this procedure, the probe will be introduced intothe thoracic cavity and positioned adjacent the epicardial layer of oneof the ventricular walls via one of a variety of conventional manners.The above electrosurgical procedure will then be performed as theelectrode is translated towards the heart until a channel is formed tothe ventricular cavity.

The system and method of the present invention may also be useful toefficaciously ablate (i.e., disintegrate) cancer cells and tissuecontaining cancer cells, such as cancer on the-surface of the epidermis,eye, colon, bladder, cervix, uterus and the like. The presentinvention's ability to completely disintegrate the target tissue can beadvantageous in this application because simply vaporizing canceroustissue may lead to spreading of viable cancer cells (i.e., seeding) toother portions of the patient's body or to the surgical team in closeproximity to the target tissue. In addition, the cancerous tissue can beremoved to a precise depth while minimizing necrosis of the underlyingtissue.

What is claimed is:
 1. A method for applying electrical energy to a bodystructure at a target site on or within a patient's body, the methodcomprising: positioning an active electrode into at least closeproximity with a body structure at the target site in the presence of anelectrically conductive fluid positioning a return electrode within theelectrically conductive fluid to generate a current flow path betweenthe active and return electrodes; and applying a high frequency voltagedifference between the active and return electrodes such that anelectrical current flows from the active electrode, through the targetsite, and to the return electrode, the high frequency voltage differencebeing in the range of about 500 to 1400 volts peak to peak.
 2. Themethod of claim 1 wherein high frequency voltage is in the range ofabout 700 to 900 volts peak to peak.
 3. The method of claim 1 furthercomprising spacing the return electrode from the body structure suchthat the return electrode does not contact any portion of the bodystructure.
 4. The method of claim 1 further comprising vaporizing theelectrically conductive fluid in a thin layer over at least a portion ofthe active electrode, contacting the body structure with the thin layerand inducing a discharge of energy to the body structure in contact withthe thin layer.
 5. The method of claim 4 further comprising ionizing thethin layer of vaporized fluid to form a plasma over at least a portionof the active electrode.
 6. The method of claim 4 wherein at least aportion of the energy is in the form of energetic electrons.
 7. Themethod of claim 1 further comprising generating a voltage gradientbetween the active electrode and the body structure at the target site,the voltage gradient being sufficient to create an electric field thatcauses the breakdown of tissue through molecular dissociation ordisintegration.
 8. The method of claim 1 wherein the return electrode isspaced a distance of about 0.5 to 10 mm from the body structure duringthe applying step.
 9. The method of claim 1 wherein the return electrodeis spaced a distance of about 1.0 to 5.0 mm from the body structureduring the applying step.
 10. The method of claim 1 further comprisingintroducing the electrically conductive fluid to the target site suchthat the active and return electrodes contact the electricallyconductive fluid and the electrically conductive fluid completes aconductive path between the active and return electrodes.
 11. The methodof claim 1 wherein the electric current flows substantially through theelectrically conductive fluid while minimizing electric current flowpassing through the body structure.
 12. The method of claim 1 wherein atleast a portion of the electric current passes through the bodystructure.
 13. The method of claim 1 wherein the active electrodecomprises a single active electrode disposed near the distal end of aninstrument shaft.
 14. The method of claim 1 wherein the active electrodeincludes an array of electrically isolated active electrodes disposednear the distal end of an instrument shaft.
 15. The method of claim 1wherein the electrically conductive fluid comprises isotonic saline. 16.The method of claim 1 further comprising wherein the return electrode isspaced from the active electrode by an electrically insulating membercomprising an inorganic material.
 17. The method of claim 16 wherein theinorganic material is selected from the group consisting essentially ofceramic, glass and glass/ceramic compositions.
 18. The method of claim 1wherein the applying voltage step is carried out by applying power tothe active electrode, wherein said power is controlled based on theelectrical impedance between the active electrode and the returnelectrode.
 19. The method of claim 18 further comprising limiting powerto the active electrode when the electrical impedance is less than athreshold value.
 20. The method of claim 1 wherein temperature at thetarget site is measured by a temperature sensor adjacent to the activeelectrode and power delivery to the active electrode is limited if themeasured temperature exceeds a threshold value.
 21. The method of claim1 wherein the return electrode has a substantially greater exposedsurface area than the active electrode.
 22. The method of claim 1wherein the return electrode has an exposed surface area at least 5times greater than an exposed surface area of the active electrode. 23.A method for applying electrical energy to a body structure at a targetsite on or within a patient's body, the method comprising: positioningan active electrode into at least close proximity with a body structureat the target site in the presence of an electrically conductive fluid;positioning a return electrode within the electrically conductive fluidto generate a current flow path between the active and returnelectrodes; and applying a high frequency voltage difference between theactive and return electrodes such that an electrical current flows fromthe active electrode, through the target site, and to the returnelectrode, the high frequency voltage being sufficient to cause themolecular breakdown of tissue cells in the body structure.
 24. Themethod of claim 23 wherein the high frequency voltage difference is inthe range of about 500 to 1400 volts peak to peak.
 25. The method ofclaim 23 wherein the high frequency voltage is in the range of about 700to 900 volts peak to peak.
 26. The method of claim 23 further comprisingspacing the return electrode from the body structure such that thereturn electrode does not contact any portion of the body structure. 27.The method of claim 23 further comprising vaporizing the electricallyconductive fluid in a thin layer over at least a portion of the activeelectrode, contacting the body structure with the thin layer andinducing a discharge of energy to the body structure in contact with thethin layer.
 28. The method of claim 27 further comprising ionizing thethin layer of vaporized fluid to form a plasma over at least a portionof the active electrode.
 29. The method of claim 27 wherein at least aportion of the energy is in the form of energetic electrons.
 30. Themethod of claim 23 further comprising generating a voltage gradientbetween the active electrode and the body structure at the target site,the voltage gradient being sufficient to create an electric field thatcauses the breakdown of tissue through molecular dissociation ordisintegration.
 31. The method of claim 23 wherein the return electrodeis spaced a distance of about 0.5 to 10 mm from the body structureduring the applying step.
 32. The method of claim 23 wherein the returnelectrode is spaced a distance of about 1.0 to 5.0 mm from the bodystructure during the applying step.
 33. The method of claim 23 furthercomprising introducing the electrically conductive fluid to the targetsite such that the active and return electrodes contact the electricallyconductive fluid and the electrically conductive fluid completes aconductive path between the active and return electrodes.
 34. The methodof claim 23 wherein the electric current flows substantially through theelectrically conductive fluid while minimizing electric current flowpassing through the body structure.
 35. The method of claim 23 whereinat least a portion of the electric current passes through the bodystructure.
 36. The method of claim 23 wherein the active electrodecomprises a single active electrode disposed near the distal end of aninstrument shaft.
 37. The method of claim 23 wherein the activeelectrode includes an array of electrically isolated active electrodesdisposed near the distal end of an instrument shaft.
 38. The method ofclaim 23 wherein the electrically conductive fluid comprises isotonicsaline.
 39. The method of claim 23 further comprising wherein the returnelectrode is spaced from the active electrode by an electricallyinsulating member comprising an inorganic material.
 40. The method ofclaim 39 wherein the inorganic material is selected from the groupconsisting essentially of ceramic, glass and glass/ceramic compositions.41. The method of claim 23 wherein the applying voltage step is carriedout by applying power to the active electrode, wherein said power iscontrolled based on the electrical impedance between the activeelectrode and the return electrode.
 42. The method of claim 41 furthercomprising limiting power to the active electrode when the electricalimpedance is less than a threshold value.
 43. The method of claim 23wherein temperature at the target site is measured by a temperaturesensor adjacent to the active electrode and power delivery to the activeelectrode is limited if the measured temperature exceeds a thresholdvalue.
 44. The method of claim 23 wherein the return electrode has asubstantially greater exposed surface area than the active electrode.45. The method of claim 23 wherein the return electrode has an exposedsurface area at least 5 times greater than an exposed surface area ofthe active electrode.